Graphic option
keyDefault value: "" (empty string)
key is the name of a function in the legend. If key is an
empty string, no key is assigned to the function.
This option affects the following graphic objects:
gr2d: points, polygon, rectangle,
ellipse, vector, explicit, implicit,
parametric, and polar.
gr3d: points, explicit, parametric,
and parametric_surface.
Example: